Self-restoring type current limiting device

ABSTRACT

A self-restoring type current limiting element comprises a first and a second terminal carried in opposite relationship in an outer hollow cylinder with the first terminal insulated from the cylinder. The first and second terminals are electrically interconnected through a vein of self-restoring type current limiting material and connected to those of a similar current limiting element through respective tubes with cooling fins filled with the current limiting material. The material in each of the elements can circulate through the other element via the tubes.

United States Patent Wada et al. 1 Aug. 26, 1975 [54] SELF-RESTORING TYPE CURRENT 3.644.860 2/1972 Yamagata et a1 .1 1337/21 3,747,040 7/1973 lnoue 337/l2l X LIMITING DEVICE [75] Inventors: Yuichi Wada; Teijiro Mori; Suenobu Hamano, all of Amagasaki, Japan [73] Assignee: Mitsubishi Denki Kabushiki Kaisha,

Japan [22] Filed: May 31, 1974 211 Appl. No.: 475,252

[52] US. Cl. 337/116; 337/21; 337/114; 337/121 [51] Int. Cl. H0lh 87/00 [58] Field of Search 337/114115, 337/1l6,117,118,119,120,121, 21, 331

[56] References Cited UNITED STATES PATENTS 3,389,360 6/1968 Keenan 337/114 3,599,137 8/1971 Ito 337/114 X Primary Examiner-J. D. Miller Asa/stun! ExuminerFred E. Bell Attorney, Agent, or FirmRobert E. Burns; Emmanuel .1. Lobato; Bruce L. Adams [57] ABSTRACT 4 Claims, 4 Drawing Figures PATENTEU Abezsms sum 1 0F 2 F/GJ (PRIOR ART) PATENTED AUG 2 61975 SHEET 2 OF 2 SELF-RESTORING TYPE CURRENT LIMITING DEVICE BACKGROUND OF THE INVENTION This invention relates to a self-restoring type current limiting device and more particularly to cooling means for such a device.

Self-restoring type current limiting devices of the conventional construction have comprised the first and second terminal members carried in opposite relationship in the outer hollow cylinder. the first terminal member being electrically insulated from the outer cylinder, a vein of self-restoring type current limiting material for electrically connecting the first to the second terminal member and buffer means disposed in the second terminal member.

Current limiting devices of the type referred to have the rated value of continuous current dependent principally upon the sum of a quantity of heat conducted from the first and second terminal member to the associated holder unit through the conduction of heat and a quantity of heat dissipated from the outer cylinder through the radiation and/or the convection of the air. Thus the conventional current limiting devices have their ratings as determined by materials, shapes etc. of structural components subject to limitations as to the properties of the material. This means that selfrestoring type current limiting devices of the conventional construction have not always met the requirements for a decrease in overall size and an increase in rated currrent capacity.

SUMMARY OF THE INVENTION Accordingly it is an object of the present invention to provide a new and improved self-restoring type current limiting device high in a rated value of current capacity.

The present invention accomplishes this object by the provision of a self-restoring type current limiting device comprising at least one pair of self-restoring type current limiting elements each including a pair of terminal members and a vein ofelcctrically conducting material disposed therein to electrically interconnect the pair of terminal members. the electrically conductive material being responsive to a flow of overcurrent therethrough to be evaporated one communication tube filled with the electrically conductive material to connect each of the terminal members of each of the current limiting element to a corresponding terminal member of the other current limiting element to permit the electrically conductive material in each of the current limiting element to circulate through the other current limiting element via the communication tubes, and cooling means disposed on each of the communication tubes to cool the electrically conductive material.

In order to increase a voltage withstood by the selfrestoring type current limiting device. a plurality of current limiting elements as described in the preceding paragraph may be serially interconnected through connecting blocks to communicate the electrically conductive material in each element with that in the adjacent elements to form one string of serially connected elements. Also a similar string is formed equal in the number ol the elements to the one string. A plurality of communication tubes lilled with the electrically conductive material connects the connecting blocks of the one string to corresponding blocks of the other string and also the end elements of the one string to a corresponding end element of the other string to communicate the conductive material in each element of one of the strings with that a corresponding element of the other strings through the associated communication tubes. As above described. cooling means may be disposed on each of the communication tubes. Also electromagnetic pump means may be operatively coupled to each of the connecting blocks of one of the strings.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more readily apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a longitudinal sectional view of a selfrestoring type current limiting element constructed in accordance with the principles of the prior art;

FIG. 2 is a longitudinal sectional view of a selfrestoring type current limiting device constructed in accordance with the principles of the present invention; and

FIGS. 3 and 4 are views similar to FIG. 2 but illustrating different modifications of the present invention.

Throughout the Figures like reference numerals designate identical or similar components.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. I of the drawings, a selfrestoring type current limiting element generally designated by the reference numeral 10 is of the conventional construction and comprises a first terminal member 12, a second terminal member 14 disposed in spaced opposite relationship with the first terminal 12 and an outer hollow cylindrical member 16 having the second terminal member 14 fixedly screw threaded into one end portion thereof. The outer cylindrical member I6 is provided on the other end portion with a stepped hole 18 into which the first terminal member 12 and an electrically insulating body 20 of ceramic material are inserted through a vitreous, electrically insulating member 22. The first terminal member 12 projects beyond the other end of the outer cylindrical member 16. The vitreous insulating member 22 is adapted to fill an annular space formed between the outer cylindrical member 16 and the first terminal member and insulating body 12 and 20 respectively in the heating and compressing steps upon producing the device.

As shown in FIG. 1, an opening 24 axially extending through one portion of the first terminal member 12 is connected to an opening 26 extending through the insulating body 20 and equal in diameter to the opening 24. Then the opening 26 is opened at a transition opening 28 extending through the remaining portion of the outer cylindrical member 16 and connected to a larger opening 30 disposed in the second terminal member 14. The communicating openings 24, 26, 28 and 30 are filled with an electrically conductive material 32 such as sodium, potassium or sodium-potassiumalloy known as a self-restoring type current limiting material. The electrically conductive material 32 electrically eonnects the first terminal member 12 to the second termi nal member I4 at room temperature.

Disposed in the opening 30 in the second terminal member 14 is a movable piston 34 defining one end of a vein formed of the current limiting material 32 and forming a pressure buffer chamber 36 between the same and the bottom of the opening 30. The bottom of the chamber 36 communicates with a small opening opened at the exposed end of the second terminal member 14 and having a sealing screw 38 screw threaded thereinto. Any suitable inert gas under a predetermined pressure fills the buffer chamber 36 and a sealing O ring 40 is disposed around the piston 34.

In the normal mode of operation, the conductive material 32 increases in temperature by means of .loules heat generated therein to be in the form of a liquid while it receives a predetermined pressure provided by the piston 34 ensuring that the first terminal member 12 is maintained in electrically conducting relationship with the second terminal member 16.

With the self-restoring type current limiting element of FIG. 1 connected in an electric circuit, an overcurrent may flow through the electric circuit for some reason. In this case, Joules heat generated in the conductive material 32 due to the flow of overcurrent therethrough causes the instantaneous evaporation of the material 32 to form a corresponding gas presenting a high resistance before the flow of overcurrent. Following this a circuit interrupter connected in the circuit is operated to open the circuit. Under these circumstances, the pressure buffer chamber 36 is operated to relieve an explosive pressure developed upon instantaneously evaporating the material 32 due to the flow of overcurrent therethrough. After the circuit interrupter has been operated to open the circuit, the evaporated material 32 decreases in temperature to be returned back to its liquid state, thereby to return the piston 34 back to its original position. Thus the first terminal member 12 is again electrically connected to the second terminal member 14.

The current limiting element as above described has a rated value of a continuous current therethrough dependent mainly upon the sum of quantity of heat conducted through the first and second terminal members 12 and 14 respectively to the associated holder unit (not shown) through the conduction of heat and a quantity of heat from the outer cylindrical member 16 released to the exterior through the radiation or convection of the air. Therefore the rating of selflrestoring type current limiting element is determined by materials and shapes of components and so on. In conventional current limiting elements of the self-restoring type such as shown in FIG. 1, the structural components have been subject to limitations as to the properties of the material. Therefore such current limiting elements have not always met the requirements for a decrease in overall size and an increase in rated current capacity.

The present invention contemplates to increase the normal current capacity of self-restoring type current limiting devices by adding thereto simple cooling means.

Referring now to FIG. 2, there is illustrated a selfrestoring type current limiting device constructed in accordance with the principles of the present invention. The arrangement illustrated comprises a pair of selfrestoring type current limiting elements a and 10!) substantially identical to the element shown in FIG. 1 excepting that the openings 24 disposed in the first terminal members 12 of both elements 10a and 10b are directly interconnected through a communication tube 40a while the openings 32 disposed in the second terminal members 14 are similarly interconnected through a communication tube 4012. Each tube 40a or 4012 is provided with a plurality of heat dissipation fins 42. The electrically conducting material 32 filling each of the current limiting elements 10a or 10b communicates with that filling the other elements 10b or 10a through the conducting material filling the communication tubes 40a and 40b so that the material can circulate through both elements 10a and 10b through the communication tubes 40a and 40b.

As the conductive material 32 is also good in thermal conductivity, Joules heat generated in the material 32 disposed in either of the current limiting elements and 10b is dissipated to the atmosphere through the heat dissipation fins 42. Therefore the arrangement of FIG. 2 can increase in a rated value ofa normal current capacity as compared with conventional current limiting elements such as shown in FIG. 1 resulting in a decrease in a ratio between a current with which the current limiting operation is initiated and the rated current. Thus the arrangement becomes distinctively higher in the ability to protect the associated equipment to be protected. In order to further increase the rated value of current, any suitable cooling means may be operatively coupled to the arrangement of FIG. 2. For example, an electric blower (not shown) may be provided to forcedly cool the fins 42.

An arrangement shown in FIG. 3 is different from that illustrated in FIG. 2 only in that in FIG. 3 an electromagnetic pump is disposed to cause the electrically conductive material to forcedly circulate through both current limiting elements and two communication tubes. As shown in FIG. 3, a pumping block 44a is screw threaded onto the first terminal member 12 of the current limiting element 10a and a connecting block 4412 is similarly connected to the first terminal member 12 of the other element 10b. Then the communication tube 40a with the heat dissipation fins 42 operatively connects the pumping block 440 to the connecting block 44b. Each block 440 is provided with a fluid passageway through which the communication tube 40a communicates with the interconnected openings disposed in the associated current limiting element 10a or 10b to form a closed loop of the conductive material 32. An electromagnetic coil 46 is operatively coupled to the pumping block 44a.

In operation, a direct current is flowing through the electromagnetic coil 46 in such a sense that a magnetic field is established in the current limiting material 32 filling the fluid passageway within the pumping block 44a and normal to the plane of FIG. 3 as shown by the symbol a circled dot." With a load direct current flowing through both current limiting elements 100 and 10b in the direction of the solid arrows A shown in FIG. 3, a driving force is generated in the direction of the dotted arrows m shown in FIG. 3 to cause the conductive material to circulate through the communication tubes 40a and 40b and the current limiting elements 10a and 10b in the same direction.

If the load current is an alternating current then an alternating current in phase of the load current is caused to flow through the electromagnetic coil 46 whereby the current limiting material circulates through the current limiting elements 10a and 10b and the communication tubes 40:! and 40b in the direction of the dotted arrows m shown in FIG. 3.

In the arrangement of FIG. 3. the electromagnetic coil 46 urges the conductive material to circulate through the tubes 40a and 40b and the elements 10a and 10b to increase the ability to cool the material. Thus the current limiting elements 10a and [0b can further increase in the rated current as compared with the arrangement as shown in FIG. 2.

FIG. 4 shows a series combination of the arrange ment shown in FIG. 2 and that illustrated in FIG. 3. More specifically. the arrangement of FIG. 2 is operatively connected to the pumping and connecting blocks 44a and 44b of the arrangement shown in FIG. 3 to form a pair of closed loops for the passage of the current limiting material 32. One of the loop is traced from the pumping block 44a through the central communication tube 40a, the connecting block 44h. the lower current limiting element [0b. the lower communication tube 40b and the lower current limiting element 10a and back to the pumping block. The other loop is traced from the pumping block 44a through the central communication tube 40a, the connecting block 44b, the upper current limiting element 100', the upper communication tube 40(- and the upper current limiting element ")0 and back to the pumping block.

Electrically. a pair of current limiting elements 100 and 10a serially interconnected are connected in parallel circuit relationship with a series combination of the two current limiting elements lOd and 10b to increase a voltage withstood by the arrangement upon limiting the current.

With the relative sense of the load current to the current for energizing the electromagnetic coil 46 remain ing unchanged from that in the arrangement of FIG. 2, the current limiting material 32 from the pumping block 44a flows through the central communication tube 401: as shown at the dotted arrows m in FIG. 4 and then is divided into two streams. One of the stream flows through the lower loop as shown at the dotted ar rows m, in FIG. 4 while the other stream flows through the upper loop as shown at the dotted arrows H1 in FIG. 4.

While a pair of current limiting elements are shown in FIG. 4 as being serially interconnected, it is to be understood that any desired number of current limiting elements may be serially interconnected to form a string of the elements with the buffer means disposed in the pumping and connecting blocks 44a and 4412 so as to project laterally as viewed in FIG. 4.

From the foregoing it will be appreciated that the present invention provides a self-restoring type current limiting device comprising communication tubes for connecting terminal members of each of a plurality of current limiting elements to corresponding terminal members of the associated current limiting element to permit an electrically conductive material filling each element to circulate through the associated element and a set of heat dissipation fins disposed on each of the communication tubes. In order to increase a quantity of heat dissipated from the fines, cooling means such as an electric vacuum cleaner or an electromagnetic pump may be provided. Therefore a larger quantity of Joules heat generated in the electrically conductive material is dissipated to the atmosphere resulting in an increase in a rated value of the current capacity.

What we claim is:

1. A self-restoring type current limiting device comprising at least one part of self-restoring type current limiting elements each including a pair of terminal members and a vein of electrically conductive material disposed therein to electrically interconnect said pair of terminal members, said electrically conductive material being responsive to a flow of overcurrent therethrough to be evaporated, one communication tube filled with said electrically conductive material to connect each of said terminal member of each of said current limiting elements to a corresponding terminal member of the other current limiting element to permit the electrically conductive material in each of said current limiting elements to communicate with that in the other current limiting element via said communication tubes, and cooling means disposed on each of said communication tubes to cool said electrically conductive material.

2. A self-restoring type current limiting device comprising a pair of strings each including a plurality of self-restoring type current limiting elements each having a pair of terminal members disposed in opposite relationship therein and a vein of electrically conductive material for electrically interconnecting said pair of terminal members, said electrically conductive material being responsive to a flow of overcurrent therethrough to be evaporated, and including a plurality of connecting blocks interposed respectively between adjacent current limiting element to interconnect the 0pposite terminal members thereof to render said electrically conductive material in each of said current limiting element continuous to that in the adjacent element. a plurality of communication tubes filled with the electrically conductive material to connect said connecting blocks of one of said strings to corresponding blocks of the other string and connect end elements of one of said string to a corresponding end element of the other strings respectively to permit said electrically conductive material in each of said current limiting elements of one of said strings to communicate with that in the mating element of the other strings through the associated communication tubes, and cooling means disposed on each of said communication tubes to cool the electrically conductive material.

3. A selfrestoring type current limiting device as claimed in claim 1 wherein electromagnetic pump means is operatively coupled to one of said communication tubes to drive said electrically conductive mate rial in a predetermined direction to circulate it through current limiting elements via said communication tubes.

4. A self-restoring type current limiting device as claimed in claim 2 wherein electromagnetic pump means is operatively coupled to each of said connecting blocks of one of said string to drive said electrically conductive material in said communication tubes in a predetermined direction to circulate said electrically conductive material through each pair of the mating elements of both strings via the associated communica tion tubes. 

1. A self-restoring type current limiting device comprising at least one part of self-restoring type current limiting elements each including a pair of terminal members and a vein of electrically conductive material disposed therein to electrically interconnect said pair of terminal members, said electrically conductive material being responsive to a flow of overcurrent therethrough to be evaporated, one communication tube filled with said electrically conductive material to connect each of said terminal member of each of said current limiting elements to a corresponding terminal member of the other current limiting element to permit the electrically conductive material in each of said current limiting elements to communicate with that in the other current limiting element via said communication tubes, and cooling means disposed on each of said communication tubes to cool said electrically conductive material.
 2. A self-restoring type current limiting device comprising a pair of strings each including a plurality of self-restoring type current limiting elements each having a pair of terminal members disposed in opposite relationship therein and a vein of electrically conductive material for electrically interconnecting said pair of terminal members, said electrically conductive material being responsive to a flow of overcurrent therethrough to be evaporated, and including a plurality of connecting blocks interposed respectively between adjacent current limiting element to interconnect the opposite terminal members thereof to render said electrically conductive material in each of saId current limiting element continuous to that in the adjacent element, a plurality of communication tubes filled with the electrically conductive material to connect said connecting blocks of one of said strings to corresponding blocks of the other string and connect end elements of one of said string to a corresponding end element of the other strings respectively to permit said electrically conductive material in each of said current limiting elements of one of said strings to communicate with that in the mating element of the other strings through the associated communication tubes, and cooling means disposed on each of said communication tubes to cool the electrically conductive material.
 3. A self-restoring type current limiting device as claimed in claim 1 wherein electromagnetic pump means is operatively coupled to one of said communication tubes to drive said electrically conductive material in a predetermined direction to circulate it through current limiting elements via said communication tubes.
 4. A self-restoring type current limiting device as claimed in claim 2 wherein electromagnetic pump means is operatively coupled to each of said connecting blocks of one of said string to drive said electrically conductive material in said communication tubes in a predetermined direction to circulate said electrically conductive material through each pair of the mating elements of both strings via the associated communication tubes. 